Xray cross complementing protein 1 (XRCC1) participates in the repair of DNA damage and, in mice, has been indicated to also be required for embryonic development. XRCC1 interacts with the DNA repair proteins poly-ADP-ribose polymerase (PARP), DNA polymerase (beta-Pol), and DNA ligase III. The complex formed between XRCC1, beta-Pol and DNA ligase III repairs incised abasic sites in the final three steps of the base excision repair pathway. The N-terminal domain of XRCC1 has been shown to bind specifically to single-strand break DNA and its structure has recently been determined by high resolution NMR methods. This proposal seeks to characterize the molecular mechanism of the interaction of the N-terminal domain of XRCC1 with DNA substrate and with beta-Pol. Mutagenesis methods, in combination with biochemical assays, will be used to elucidate those residues that contribute to binding. The existing NMR-based structure and chemical shift perturbation map of the binding surface will guide the mutagenesis. The full range of DNA substrate specificity will also be explored. Backbone dynamics will be characterized and correlated with functional binding sites. The 40 kDa complex, involving the N-terminal domain of XRCC1, DNA substrate and beta-Pol, will be structurally characterized using NMR-based methods.